For many years, attempts have been made to develop optical methods for measuring particles, i.e. sludge, in fluids. Within this context, fouling or contamination of a sampler or tester is a serious problem that risks making the measurement result entirely or partly useless after a certain operating time. The development of optical testers has therefore included different alternative methods of cleaning the tester, such as the use of wind-screen wipers on the glass surface transmitting the optical radiation, water flushing to keep the surface clean and the use of a brush for mechanical or manual cleaning. No existing testers have worked satisfactorily, but there has been clogging and in the case of mechanical cleaning scratches have often developed on the measuring surface, which has lead to unreliable measurement results.
The testers have throughout been positioned down in a tank containing sludge or in a line containing sludge, which has made the tester difficult to reach for inspection and attendance, for instance by mechanical cleaning.
As a consequence, this measuring technique has not been working properly and it has not been possible to measure sludge content in a reliable manner.